On the 50th anniversary of the passing of a key countryside law, environmental campaigners say the legislation is failing to give proper protection to the sites of greatest scientific interest.

The 1949 National Parks and Access to the Countryside Act established the United Kingdom's system of national parks and areas of outstanding natural beauty, designed to give special protection to some of the best loved landscapes.

Damage to SSSIs when the Newbury bypass was built sparked protest

It also created SSSIs - sites of special scientific interest - widely reckoned to be the jewels in the crown of nature conservation.

There are almost 6,500 SSSIs in England, Scotland and Wales, covering about 8% of the surface of Britain. They are chosen for their biological or geological interest.

They range in size from the Wash, at 66,000 hectares, to more than 130 sites of less than half a hectare. Most are in private hands.

They are designated by the government's conservation advisers, English Nature, Scottish Natural Heritage, and the Countryside Council for Wales. There is a parallel but slightly different system in Northern Ireland.

SSSIs are said to "form a national network of areas representing in total those parts of Great Britain in which the features of nature, and especially those of greatest value to wildlife conservation, are most highly concentrated or of highest quality".

But the SSSI system has one key flaw - it designates the best sites, but cannot guarantee them protection.

The nuclear power station at Dungeness is on an SSSI

Friends of the Earth (FoE) says that since the Labour government took office two years ago, "SSSIs have suffered damage at a rate of almost one a day".

A main concern is the voluntary principle underpinning the system, which means that landowners face at most a small fine if they damage or destroy a site without consulting the relevant conservation adviser.

An FoE study of 50 sites, "SSSIs: RIP", details some of the shortcomings it says the system allows:

More than 300 SSSIs in the UK are damaged every year

45% of sites (at least in England) are judged to be in an "unfavourable" condition

10 habitats are in severe decline, with 97% of wildflower meadows, for example, lost between 1934 and 1984

25 species have become extinct, including birds, butterflies and moths, fish, plants and one bat species

FoE, the World Wide Fund for Nature and other groups want the government to introduce a countryside Bill which will include better site protection.

But they say they fear a bill drafted by the Environment Department has yet to be included in the 1999 Queen's Speech.

One MP supporting their call, Helen Brinton, said: "We urgently need to live up to our promises and deliver new laws."

"If we do not demonstrate a positive, popular agenda for the countryside, mine and other Labour MPs' seats will go."